This invention relates generally to frequency regulation techniques and, more particularly, to precise frequency control of power waveforms.
The American electric power generation and distribution system is a very stable system in regard to frequency control. Historically, the frequency fluctuations have always been small, rarely exceeding 0.1 Hz. As a consequence, American electronic equipment designers assume that a reliable, closely regulated source of power is available to the user. This is usually true in the continental United States. However, when American-made electronic equipment is used in foreign countries, and in particular, small foreign countries where the power systems are often overloaded and poorly regulated, it does not function properly. Power system frequency fluctuations as large as 2 Hz have been observed in at least one small foreign nation, resulting in the Navy station there having to switch completely to standby diesel generators as often as twice a day. In addition, many European countries use 50 Hz powder instead of the 60 Hz American standard. Therefore, for American military equipment deployed world-wide, power frequency converters are often required to fulfill the assigned mission.
In the conventional power frequency converter system, unregulated AC power is first converted to DC power by an input rectifier and low-pass filter combination. This DC power is then regulated by a DC switching regulator and converted to AC power at the desired frequency by a power switching inverter. Power switching in the regulator and inverter are necessary to minimize power losses in the system. A low-pass filter on the output removes harmonic distortion generated in the power inverter. The desired frequency stability is achieved by driving the power inverter with a crystal oscillator. The desired output amplitude regulation is obtained with the DC switching regulator. The feedback signal for this regulator is derived by rectifying and filtering the system output voltage. This feedback signal is compared to a DC reference in the regulator to obtain an error signal to control the regulator chopper duty cycle.
The major problem associated with the conventional system is the size of the inductors and capacitors required for input and output low-pass filters. The size of the output filter can be greatly reduced but only at the expense of a great increase in the complexity of the power inverter. For example, the simplest type of power inverter is the push-pull square wave inverter which requires a relatively large output filter to remove the odd harmonics in the square wave. More elaborate power inverter circuits can be used to eliminate or reduce the undesired harmonics and thereby reduce the size of the output filter required.